Glioblastoma multiforme (GBM) remains one of the most incurable human malignancies. The objective of the proposed research is to test the therapeutic potential of genetically engineered herpes simplex viruses (HSV) combined with ionizing radiation for the treatment of GBM. Our preliminary results indicated that HSV mutants prepared by Dr. Roizman's group (Project 1) enhanced tumoricidal effects of ionizing radiation on malignant glioma xenografts. There are three specific aims: Firstly, we will test the hypothesis that genetically engineered HSV enhance glioma cell killing by ionizing radiation by comparing effectiveness of radiation alone, HSV alone and combined treatment in human glioma xenografts grown in the nude mouse hind limb. The second aim is to test the hypothesis that interaction between genetically engineered HSV and radiation is dependent on an immune response or the production of cytokines. We will use the immunocompetent C57BL/6 mouse bearing syngeneic Gl-261 gliomas and will determine the nature of inflammatory infiltrates following treatment. The relative importance of each will be established with neutralizing antibodies to deplete specific subsets of leukocytes identified. Cytokine responses in glioma cells will also be determined following treatment and neutralizing antibodies will be used to determine relative contribution of cytokines to the radiation/HSV mediated tumor regression. The third aim is to test the hypothesis that the radio enhancing effects of genetically engineered HSV can be further enhanced by delivery of foreign genes encoding cytokines or pro-drug converting enzymes. Initial studies will involve the gene for the pro-drug converting enzyme cytosine deaminase (CD) which will be inserted into HSV and delivered to glioma xenografts. Non-toxic 5-flurocytosine will be administered systematically and converted to 5-fluorouracil in tumor cells expressing CD. Foreign gene inserts will be linked to both radiation inducible promoters as well as constitutive promoters. We propose that genetically engineered HSV will enhance the tumoricidal effects of therapeutic radiation and that HSV- vectored foreign genes that encode cytotoxic proteins will further increase the radio-sensitizing effect in malignant glioma cells.